Recently, as personal computers and television receivers have become lighter and thinner, reduction in thickness and weight of display devices has also been wanted. In answer to such demands, flat panel type displays such as liquid crystal displays (LCDs) have been developed in place of cathode ray tubes (CRTs).
An LCD is a display device which produces desired image signals by applying electric fields across a liquid crystal layer having anisotropic dielectric constants, injected between a pair of substrates so that the strength of the electric fields is controlled to thereby control the amount of light passing through the substrates. Such LCDs are typical examples of handy flat panel type displays. Of these, TFT LCDs that employ thin-film transistors (TFT) as switching elements are mainly in use.
Lately, since TFT LCDs have been not only used as the display devices of computers but also used widely as the display devices of television receivers, the need for rendering motion pictures has been increased. However, since the conventional TFT LCDs are low in response speed, they have a drawback that it is difficult to reproduce motion pictures.
In order to make the liquid crystal response speed problem better, there is a known liquid crystal driving method wherein in accordance with the combination of the input image data of the previous frame and the input image data of the current frame, either a higher (overshot) drive voltage than the predetermined gray scale level voltage that corresponds to the input image data of the current frame or a lower (undershot) drive voltage is supplied to the liquid crystal display panel. In this specification of the present application, this driving scheme should be defined as overshoot drive.
FIG. 7 shows a schematic configuration of a conventional overshoot drive circuit. Specifically, the input image data (current data) of the N-th frame being about to be displayed and the input image data (previous data) of the (N-1)-th frame being stored in a frame memory 1 are loaded into a write-gray scale level determining portion 2, wherein the patterns of the gray scale level transitions between both the data and the input image data of the N-th frame are checked with an OS table memory (applied voltage data table) 3 stored in an external memory, so that the write-gray scale level data needed for image display of N-th frame is determined based on the applied voltage data that is obtained from the checkup, and is supplied to a liquid crystal display panel 4.
With a general liquid crystal display panel, there has been a problem in that it takes long time to make a transition from a certain half gray scale level to another half gray scale level, so that it is impossible to display the half gray scales within the period of one frame (e.g., 16.7 msec. for a case of progressives can of 60 Hz). This not only produces afterimages but also hinders correct half gray scale display. Use of the above-described overshoot drive, however, enables display of the aimed half gray scales within a short time (one frame period) as shown in FIG. 8.
Here, since the above-described overshoot drive scheme is implemented on the basis that the liquid crystal display panel is always able to display the target gray scale level after one frame period for every the gray scale level transition, the input image data of the previous frame is directly input as the previous data to write-gray-scale level determining portion 2. However, if the liquid crystal has a poor response characteristic or when the data is constructed of 8 bits representing 256 gray scale levels, there has been a problem in that the response of the liquid crystal to the gray scale level of the current frame image data cannot be sufficiently compensated if the gray scale level of the previous frame image data is zero and transits to a half gray scale level.
To deal with this problem, for example, Japanese Patent Application Laid-open Hei 7 No.20828 proposes a liquid crystal display as shown in FIG. 9. This liquid crystal display includes: a signal processor 11 that processes an input image signal Xn so as to compensate the transmittance response characteristic with respect to the voltage applied to the liquid crystal; and a response predictor 12 that implements a low-pass filtering process that approximates the voltage response characteristic of the liquid crystal, on the output Zn from this signal processor 11 and feeds back the output signal Yn-1 as a corresponding predicted value for the liquid crystal response voltage to signal processor 11.
The above response predictor 12 approximates the liquid crystal response characteristic, using a set of low-pass filters (LPFs) with which the coefficient α is varied depending on the voltage level. With this method, the actual liquid crystal response voltage at the previous field can be approximated by the LPF output, therefore this voltage can be used as the initial voltage (previous data) at the next field so as to be able to compensate for the liquid crystal optical response characteristics in a more faithful manner.
In the case of the above-described conventional liquid crystal display described in Japanese Patent Application Laid-open Hei 7 No. 20828, the actual liquid crystal response voltage (displayed gray scale level) of the next field is calculated by approximating the liquid crystal response characteristic based on the voltage-level dependent LPF set. Nevertheless, as also understood from FIG. 10, for example, it is known that a typical liquid crystal display panel presents irregular response characteristics depending on the combination of the gray scale level before change (transition) and that after change (transition), and the response speed markedly lowers at transitions between particular gray scale levels.
In sum, it is impossible to obtain the exact achievable gray scale levels after a lapse of the one vertical display period for all the gray scale level transition patterns, by approximating the liquid crystal response characteristic based on the voltage level dependent LPF set as disclosed in Japanese Patent Application Laid-open Hei 7 No. 20828, hence the response and fidelity of liquid crystal to the display of motion pictures including half gray scales cannot be fully compensated for and the problem is not yet solved.
The present invention has been devised in view of the above problem to provide a liquid crystal display which, if any type of gray scale level transition occurs from one vertical display period to the next, or even if the gray scale level does not reach the aimed gray scale level within one vertical display period, is able to correctly inhibit generation of afterimages and present correct display of half gray scales even for motion pictures containing any kind of gray scale level transition pattern, by implementing overshoot drive using actual achievable gray scale levels within the one vertical display period.